Forest Health Issues in Northeast Colorado

The good news for northeast Colorado is that no new forest health issues topped the list for 2024, but the bad news is the perennial issues continue to spread, weakening and killing trees.

Counties most impacted in northeast Colorado in 2024: Clear Creek, Gilpin, Jefferson, Park

Long-time residents of northeast Colorado may be familiar with mountain pine beetle (Dendroctonus ponderosae). This native insect drew a great deal of public attention when it decimated 80 percent of pine forests in Colorado from 1996-2014.

Hillsides in Colorado impacted by the mountain pine beetle — Mountain pine beetle killed the brown trees scattered across these hillsides along Interstate 70 in Gilpin County. Photo: Dan West, CSFS

While outbreaks of the beetle have not returned to epidemic levels, the mountain pine beetle has grown its footprint in localized areas of pine forests over the past several years, spurred by drought in dense stands with low species diversity. Please refer to the map of insect and disease activity with current data from last year’s aerial survey to see where the beetle is impacting forests.

Early detection is key to managing the mountain pine beetle. It is mostly impacting ponderosa pines in this part of the state, so residents with ponderosa pines on their property should keep an eye out for signs and symptoms of infestation. Colorado State Forest Service foresters may recommend a variety of management options for landowners and communities, ranging from semiochemical packets to ward off insects from attacking trees to removing infested trees to spraying insecticides.

Education is an important strategy for managing the mountain pine beetle. The CSFS offers a fact sheet, poster and storybook available to order online.

Counties most impacted in northeast Colorado in 2024: Adams, Arapahoe, Boulder, Broomfield, eastern Jefferson, Larimer

In 2024, Fort Collins, Loveland, Lakewood and Centennial all joined the list of cities on the northern Front Range with a detection of emerald ash borer (EAB, Agrilus planipennis). From the epicenter of its first detection in Boulder in 2013, EAB has spread like a wave cresting over the Denver Metro area. It is safe to say any community in the Denver area should be on the lookout for EAB.

An emerald ash borer in bark from an ash tree — Adult emerald ash borers emerge through this piece of bark removed from an infested ash tree in Lakewood. Photo: City of Lakewood

Residents with ash trees should know the signs and symptoms of an infestation of this destructive, non-native insect. Suspected detections should be referred to the nearest Colorado State Forest Service or CSU Extension office for confirmation from experts at CSU. Ash trees comprise roughly 15 percent of all urban trees in Colorado, making this a major pest of concern for urban forests.

The CSFS offers resources and guides to help residents and arborists detect, treat and plan for EAB, including management resources for smaller communities. Denver residents can also check out the city’s Be A Smart Ash program for resources, including a free tree to replace their ash.

Counties most impacted in northeast Colorado in 2024: Boulder, Douglas, Jefferson

For a second straight year, the Douglas-fir tussock moth (Orgyia pseudotsugata) made the list of forest health issues for northeast Colorado, particularly the areas of Jefferson and Douglas counties that were affected by an outbreak of this insect in 2014-2016. This is not surprising; outbreaks of this native moth tend to be isolated and cyclical in Colorado. The moth is also present in Boulder County during this current outbreak.

Douglas-fir tussock moth pupae and eggs — Pupae and eggs of the Douglas-fir tussock moth are clustered around the door of a home in Boulder County during the outbreak in 2014. Photo: CSFS

Foresters in this part of the state have noticed more Douglas-fir trees growing in Front Range forests, resulting in more food for this insect. During its larval stage, the Douglas-fir tussock moth feeds on the needles of Douglas-fir, Engelmann spruce and occasionally blue spruce in urban areas. This weakens the trees, making them susceptible to attack from deadly bark beetles.

Concern about the moth’s impact extends beyond the health of trees to humans. Douglas-fir tussock moth larval hairs can cause skin irritation and an itchy rash, a condition known as “tussockosis,” affecting landowners and crews working in affected areas. This is problematic when in their growth stage as caterpillars, these pests migrate to homes and other buildings, especially around door and window frames, seeking sites to build their cocoons.

The CSFS offers a quick guide and treatment options for managing the Douglas-fir tussock moth.

Counties most impacted in northeast Colorado in 2024: Douglas, Jefferson, Park

The western spruce budworm (Choristoneura occidentalis) is most active in Douglas-fir trees in western and southern Jefferson County and northeast Park County. Small, isolated pockets of the budworm may also exist in Larimer and Gilpin counties in this area of Colorado.

Needles damaged by the western spruce budworm — Western spruce budworm consumed the needles on the tips of these branches. Photo: Dan West, CSFS

During its caterpillar stage, the western spruce budworm eats the needles of Douglas-fir, true fir and spruce trees. This defoliation weakens the trees, making them susceptible to attack from the Douglas-fir beetle, which will subsequently kill the trees. Despite years of spotty budworm activity in these parts of northeast Colorado, Douglas-fir beetle has not been a significant issue so far.